1. Field of the Invention
The present invention relates to a method and apparatus for chemical analysis by pyrolysis and thermal desorption techniques, and more particularly to a method and apparatus for Curie-Point Pyrolysis and/or desorption of complex, nonvolatile test samples.
2. Prior Art
Pyrolysis techniques have long been applied in analysis of macromolecules, particularly for complex molecular structures. In simple terms, the process involves the thermal fragmentation of the molecule into characteristic component parts. Analysis of these smaller molecules by standard techniques such as mass spectroscopy (MS) and/or gas chromatography (GC), develops predictable test results when performed under similar conditions. In other words, pyrolysis of a given compound at a given temperature and controlled environment will produce the same family of component molecules in a predictable manner. Such results can be readily evaluated from a mass spectrum or chromatogram.
Curie-Point pyrolysis has provided a relatively simple technique for reproducing a specified temperature for the fragmentation reaction and utilizes a wire comprised of ferromagnetic material to position a coating of the sample within a reaction zone. See, for example, U.S. Pat. No. 4,408,125 by Meuzelaar. At the Curie-point of a temperature range, the ferromagnetic material loses its magnetic properties and therefore fails to inductively absorb energy from an associated electromotive source. Peak temperature of the wire is accordingly stabilized at the Curie-point temperature, providing reproducible character to the reaction zone. Ideally, predictable fragmentation products can then be produced for identical samples, assuming a common temperature/time profile.
The degraded components of the reaction are then conducted to an inlet of the analysis device for detection. Generally, the pyrolysis apparatus is integrated with the analysis system (GC, MS, etc) in a manner such that fragment transfer is reproducible with similar repetitive efficiency and accuracy as is the pyrolysis reaction. Failure to control sample transfer can materially alter output of the detection device, as well as mask the sample results with unwanted background detection.
The prior art has suggested that apparatus design provide for total transmission of volatile products for analysis. Efficient transmission of volatile products requires (a) rapid removal from the reaction zone to avoid secondary reactions (e.g., secondary pyrolysis or recombination reactions), (b) minimization of condensation losses on the reactor walls and (c) rapid transfer into the detecting instrument (e.g., gas chromatograph and or mass spectrometer). These general principles are well recognized in modern textbooks, on analytical pyrolysis such as: Analytical Pyrolysis, William J. Irwin, Marcel Dekker, Inc., (1982) p49. This author suggests that in pyrolysis mass spectrometry, the pyrolyzer should be within the ion source of the mass spectrometer if at all possible. Similarly, GC procedures should involve minimization of gas diffusion by conducting pyrolysis as near to the column top as possible and with the unit located within the column heater zone to avoid condensation onto cool surfaces.
Most of the above design considerations with regard to efficient and rapid sample transfer also apply when the objective is to desorb intact, low volatile compounds rather than to effect pyrolysis. In fact, many complex samples consist of a mixture of more or less volatile components which can be thermally desorbed and nonvolatile components which need to be pyrolyzed in order to enable transfer to the detector.
A typical example of the use of a Curie-point heating device to effect thermal desorption of low volatile components is described by J. de Leeuw et al. (Analytical Chemistry, 49 (1977), 1981). The advantages of Curie-point desorption over conventional sample injection techniques using a dilute solution of the sample in a suitable solvent are: elimination of the solvent (which may adversely effect operation of the column and detector) and more rapid desorption of components (due to the high heating rates obtainable by Curie-point techniques).
Based on the aforementioned guidelines, the system design criteria adopted within the prior art have been based on coupling the reaction chamber to a receiving chamber which joins the detector inlet to the system. FIG. 1, for example, represents a basic design for implementing the above recommendations for a capillary type system. Id. p 64. This apparatus comprises a cap 1 enclosing an opening of a quartz tube 2 which receives the ferromagnetic element with coated sample, the enclosure being completed by a mounting flange 3. This area is sealed at an upper end by an O-ring 4 to a Delrin (TM) mount, which is further sealed to a pyrex tube 7 and additional Delrin mount 8 to filament enclosure for the water-cooled induction coil 6. A Teflon (TM) seal 9 abuts a lower mounting flange 10 which couples the pyrolysis component to a GC injector 11 and enclosed capillary column 12. The efficiency of this disclosed system is achieved by gas flows being routed through and around the pyrolysis tube before entering the capillary column. As with earlier designs, the object of this device is to transport all of the pyrolyzed material into the GC column. The described gas flow permits control of flow rates for the pyrolysis reaction, as well as for the flow separation within the GC column.
A particular problem arising with this conventional design is inherent with the physical and dimensional requirements of the Curie-Point reactor system. For example, the dimension of the reaction chamber must be sufficiently large to permit insertion of the sample-coated, ferromagnetic element or wire. This element must be free of contact with the inner walls of the tube forming the reaction chamber. Even though this chamber should be designed as small as possible to minimize dead space, a 2 millimeter inner diameter is necessary for use with a 0.5 millimeter inserted element. Reduction of the element dimension is difficult for reasons of heating requirements utilizing high frequencies of a typical induction source.
A principle difficulty is how to match this larger diameter, with the 1/2-1/10 millimeter internal diameter for the capillary tube which is to receive the pyrolysis desorption products. In summary, the resulting problem is how to couple a relatively wide reaction chamber tube efficiently to a very narrow capillary tube.
One of the problems arising because of this mismatch is illustrated in FIG. 2. Items 20 and 21 represent capillary columns which correspond to item 12 in FIG. 1. If the pyrolyzed sample comprises components A and B occupying the volume A+B in FIG. 2a and is transferred in full into the capillary tube, chromatographic separation over the distance "x" might be illustrated by the three components A', A'+B', and B' wherein A' and B' partially overlap at A'+B'. If, however, the size of the plug within the tube is reduced in volume, operation of separation over the same distance "x" results in resolution of the components A' and B' into detectable peaks as illustrated in FIG. 2b. An option to increasing the separation is to increase the length of the tube to allow time for the components A and B to fully separate, say at a distance of 2x. Unfortunately, in many applications, time may be of the essence. A reduction in several minutes may be critical. Moreover, increased length of the column requires a higher pressure differential across the column in order to maintain adequate column flow. This in turn results in less ideal separation conditions as well as increased pressures in the reaction zone.
In addition to the problem of maintaining a small volume of sample in order to keep separation time low, current systems encounter a variety of problems which relate to contamination of pyrolyzed/desorbed samples. Such contamination can occur by change of composition within the chamber and flow line, loss of sample by condensation of pyrolysis/desorption components on tube walls and related factors that alter the detection output by increasing background signal.
An example of contamination within the flow line is the volatization of condensate remaining from prior reactions on the tube walls of the reaction chamber and flow line. Although the reaction zone may easily reach temperatures in excess of 500 degrees C., the surrounding tube walls will generally have to remain at less than 300 degrees C. in order to prevent secondary pyrolysis of reaction products. It is not uncommon, therefore, for components of the pyrolyzed/desorbed samples to condense on the cooler wall surface subsequent to pyrolysis. Any condensate not removed after previous analysis may again volatize and mingle with the new sample pyrolysis/desorption components. Obviously, the occurrence of such prior matter will give a false reading and seriously undermine the analytical value of the system. Primary attempts to solve this problem have focused on minimizing condensation and providing better control of downline reaction environment and hardware.
Unfortunately, each addition of complex hardware not only increases cost of the equipment, but adds to the complexity of operation and possibility of component failure. What is needed is a simple technique for reducing these adverse consequences within a pyrolysis/desorption system without slowing down the speed of analysis or increasing cost and complexity.